In monitoring the internal blood pressure of a patient, one end of a catheter is inserted through the internal passageways of the blood circulation system until it reaches a desired point, and the other end is coupled by a pressure dome to a pressure transducer. The catheter and dome are filled with saline fluid so as to transmit the blood pressure to the transducer. In order to prevent occlusion of the inner end of the catheter by blood clotting, it has been found necessary to continuously provide a slow flush of saline solution. In spite of this, occlusion can occur after some time has passed so that it has been found necessary to remove any coagulation by periodically providing a fast flush of saline solution. Whereas the slow flush is so low as to have no deleterious effect on the patient, fast flush is high enough to be harmful if permitted to occur for more than a short time. For this reason, it is essential that the fast flush be provided only when the valve is held in the fast flush position by a technician and that the valve automatically reverts to providing a slow flush when the technician releases it.
Systems for providing both slow and fast flush have been devised in which the slow flush is obtained by connecting a source of pressurized saline fluid to the catheter via a tube having a small bore, and the fast flush is obtained by connecting a tube of large internal diameter in parallel with the small bore tube. The problem with such an arrangement is that if the small bore tube becomes clogged, there is no way to unclog it.
Another disadvantage of such a device is that it entraps air that interferes with the accuracy of the pressure measurements being made. The entrapment of air results from the necessity of providing pockets in the passageways that house and couple the tubes.